1. Field of the Invention
The invention relates to novel asymmetric molybdenum dialkyldithiocarbamates, prepared from asymmetric dialkylamines, which are useful as additives in lubricating compositions.
2. Discussion of the Prior Art
Molybdenum dithiocarbamate are well-known additives for lubrication formulation. Molybdenum dialkyldithiocarbamates prepared from symmetrical dialkylamines and mixtures of symmetrical dialkylamines are currently used as additives in lubricating oils (as well as mixtures thereof) for imparting antifriction properties. While these compositions are quite effective, there is a desire for compounds that will offer improvements or provide even greater lubricating properties. Also, in certain lubricating oil compositions, prior art molybdenum dialkyldithiocarbamates do not have the desired solubility, especially at lower temperatures, so that the compound separates from the lubricating composition, resulting in the formation of a haze, cloud or precipitate in the lubricating composition, thus reducing its effectiveness.
Therefore, there is also a desire for molybdenum dialkyldithiocarbamates with improved solubility in lubricating compositions. In addition, current molybdenum dithiocarbamates (DTCs) require the addition of substantial amounts of diluent oil to lower the viscosity of the additive to acceptable levels for handling. This reduces the activity of the additive by dilution. Therefore, additives that require less diluent oil to afford a readily pourable product are advantageous. Molybdenum dithiocarbamates can be rendered according to the following formula (1):

A particular type of asymmetric molybdenum dithiocarbamate (Mo DTC) is known, in which the R1 and R2 group on one side of the molybdenum groups differ from the R3 and R4 groups on the other side. However, these compounds are believed to be formed by a mixture of two different symmetric dialkylamines, HNR1R2 and HNR3R4, where R1=R2 and R3=R4, and R1 is not the same as R3. For example, U.S. Pat. No. 5,627,146 to Tanaka, et al. describes lubricating oil compositions that contain specifically C8 dithiocarbamyl and C13 dithiocarbamyl ligands to produce an asymmetric type molybdenum dithiocarbamate as an essential component thereof.
A further Tanaka patent U.S. Pat. No. 6,245,725 does speak generally of asymmetric Mo DTC compounds wherein R1, R2, R3 and R4 must merely have one dissimilar alkyl to establish asymmetry, and also states generally that compounds may be formed from a secondary amine having different hydrocarbon groups. Furthermore, the patent suggests that asymmetric Mo DTC's should be blended with symmetric Mo DTC's in order to achieve good performance. However, such a compound would have been an impossibility at the time of Tanaka's teaching, since such asymmetric dialkylamines did not exist at that time. Only recently have such amines become available, such as (2-ethylhexyl) tridecylamine from Nova Molecular Technologies, Inc. of Janesville, Wis., USA; and isodecyloxypropyl-isopropylamine and alkoxypropyl-isopropyl (alkyl=C12 to C15), from Tomah Products, Inc. of Milton, Wis., USA.
Accordingly, it is seen that the only working example in the '725 patent is the same C8/C 13 Mo DTC as taught by the Tanaka '146 patent. There is no suggestion that such asymmetric dialkylamines were available at that time, and the patent merely relies on two different secondary amines, one C8 and the other C13. Since it would not have been possible to even formulate such compounds at that time, it could also not have been foreseen that, not only are such compounds (based on asymmetric dialkylamines) possible, but that they surprisingly provide even greater friction performance than either symmetric Mo DTCs or the asymmetric Mo DTC taught by Tanaka which is formed from two different symmetric dialkyl amines.
Equally surprising is the discovery that the novel asymmetric compositions have lower viscosity than the composition taught by Tanaka. As less diluent oil is needed for handling, the result is that a lubricating composition, such as a base oil composition, can comprise a greater amount of molybdenum than can the prior art compounds. Thus, the inventive compositions allow for a lubricating composition having up to 12-15% weight percent molybdenum, compared to only about 7.5% maximum when using a symmetric Mo DTC.